Internal combustion engines, and particularly “downsized” engines, require high-pressure at the air intake to the engine cylinders. A turbocharger is a type of forced induction system used with internal combustion engines. Turbochargers deliver compressed air to an engine intake, allowing more fuel to be combusted, thus boosting the horsepower of the engine without significantly increasing engine weight. Thus, turbochargers permit the use of smaller engines that develop the same amount of horsepower as larger, normally aspirated engines. Using a smaller engine in a vehicle has the desired effect of decreasing the mass of the vehicle, increasing performance, and enhancing fuel economy. Moreover, the use of turbochargers permits more complete combustion of the fuel delivered to the engine, which in turn reduces undesirable engine emissions.
However, pressurizing the intake air raises its temperature, which reduces charge density, thereby reducing the advantage of a pressurized air intake. High charge temperature is also known to increase emissions of nitrous oxides from the engine. For these reasons, it is desirable to reduce the temperature of the charged intake air.
Engine systems may employ variable valve timing in which the timing of the movement of the engine cylinder valves is controlled and varies based on engine operating conditions. For example, a valve timing scheme referred to as “Miller timing,” in which the air intake valve closes after the piston passes bottom dead center, can be used at high engine loads, and normal timing, in which the air intake valve closes when the piston is bottom dead center, can be used at partial engine load conditions. As a result, there is a need to control the pressure and temperature characteristics of the engine air intake over an even wider range.